This invention relates to vehicle engine idle speed control systems. Such systems appear, on some engines, to offer the possibility of improved fuel economy by accurately controlling idle engine speed to the lowest speed consistent with engine and vehicle operability, safety and emission goals while providing for increases in said speed when conditions require.
Such a system may have a movable idle stop and a switch of some sort which indicates contact between the idle stop and some member of the engine throttle mechanism. The movable throttle stop may be positioned by a stepper motor and a closed loop control system which tends to maintain the engine throttle position during idle in accordance with a preset condition. For instance, a number of preset idle speeds corresponding to the desired idle speeds under a number of different engine operating conditions could be stored in a memory and called out to the closed loop idle speed control system in accordance with the sensing of said other engine operating conditions. The closed loop system would repeatedly or continuously measure engine speed, compare it with the reference and actuate the stepper motor to adjust the throttle to decrease the error between actual and desired engine idle speed.
A complication occurs, however, during vehicle coastdown from a high vehicle speed. During this mode of operation, the throttle is generally in idle position, so that the idle speed control system is switched on. However, during most of the coastdown, the coasting vehicle is driving the engine at a higher speed than the preset idle speed. The simple closed loop control system described above does not "know" that it cannot control idle speed under these circumstances; and in the attempt to exert such control, it may close the throttle completely to the closed limit position. It has been the experience of those skilled in the art that some engines may experience a significant increase in the emissions of hydrocarbons when operated under such conditions; and that the way to reduce such emissions is to increase the amount of throttle opening. Devices and systems to crack the throttle open under certain circumstances have been proposed in the past. Such systems generally have comprised a solenoid or vacuum motor to crack the throttle open by a fixed amount upon detection of manifold pressure below a certain reference. Devices of this sort often work well within their design limit; but they do not allow sufficient control over throttle position to optimize this position with regard to all the often conflicting goals of fuel economy, engine braking and emissions throughout the entire vehicle coastdown.
It is an object of this invention to provide a vehicle engine closed loop idle speed control system which permits optimization of engine throttle position during the full range of vehicle coastdown to optimize factors such as engine fuel economy, engine braking and engine emissions.
It is a further object of this invention to provide a closed loop idle speed control system for a vehicle engine which will maintain engine throttle position during a vehicle coastdown according to a predetermined schedule as engine speed decreases.